1. Field of the Invention
The invention relates to the application of dielectric, thermal insulation and other polymeric coatings to electrical cable protective sleeving and other inert solid substrates, and to the resulting coated articles.
2. Discussion of the Prior Art
In electrical cable and conduit the conductor commonly is surrounded by a protective or reinforcing braided glass fiber sleeve which has been externally coated with a suitable polymeric material such as a polyacrylate, a polyvinyl chloride, a fluoropolymer, a silicone rubber, or the like. This polymeric coating, in some instances in conjunction with the inert substrate, serves as a dielectric and/or thermal insulator, and when colored may also serve as an identifier.
The polymeric coating is applied to the glass fiber sleeving or other inert substrate initially in the form of an aqueous emulsion of the polymerizable pre-polymer or monomer, and then the polymerizable material is cured or polymerized in situ through the application of heat to the coated sleeving. However, before final polymerization or curing of the monomer or prepolymer, the water in the aqueous emulsion coating must be substantially removed, i.e., the emulsion coating must be dried and coalesced, or "devolatilized," so that polymerization will produce the desired physical and chemical results. Care must be taken in this devolatilization step to avoid conditions which would result in imperfections in the polymer coating and lead to the ultimate dielectric or thermal breakdown of the product in its end use. The avoidance of such conditions requires careful control particularly over the rate at which the water is removed in the devolatilization phase.
Devolatilization heretofore typically has been accomplished by passing the emulsion-coated sleeving or other substrate through an elongated electric, or gas- or oil-fired, hot-air devolatilization oven. Limitations on rate of water removal, and therefore on temperature, in the devolatilization zone necessitate a substantial residence time in the oven. Also, and particularly in the case of coated sleeving products intended to have high dielectric or thermal insulation quality, it may be necessary to apply the total coating in several separate layers, each of which must be devolatilized in separate passes through the oven. The necessary high residence time in the oven in turn requires either that the oven be of extreme length or that the coated substrate be subjected to several passes through the oven. Thus, the devolatilization oven may be a single-pass oven or a multiple pass oven, but multiple coat, multiple pass ovens are more common because they are smaller and less costly to construct. After dewatering, the dry coated substrate enters a suitable curing oven or zone, in which the coated substrate is heated in order to complete polymerization of polymerizable or otherwise reactive material in the coating.
From the foregoing description it is apparent that conventional devolatilization processes are expensive and have other serious drawbacks.